This invention relates to nickel-titanium alloy sputter targets for use with magnetron sputtering systems to deposit nickel.
With magnetron sputtering, magnets are located behind the cathode target in a manner as to cause closed magnetic field loops to cut through the cathode. A portion of the magnetic field loop is located adjacent the front face of the cathode. The combination of magnetic field and electric field causes electrons to spiral in long confined paths giving rise to a very dense plasma immediately adjacent to the face of the target material. This dense plasma facilitates an increased yield of material sputtered from the target.
One limitation to magnetron sputtering, however, is that this technique is not amenable to the deposition of ferromagnetic materials. A target of ferromagnetic material acts as a shunt and prevents magnetic field lines from cutting through the target and being located, as required, in front of the target. Therefore, materials such as iron and nickel cannot generally be magnetron sputtered. In order to light and maintain a plasma during the sputtering of ferromagnetic materials, such as pure nickel, it usually is necessary to limit the thickness of the nickel target to usually less than 3 mm. This thin target, however, provides only limited source material and thereby reduces the useful life of the target.
Some limited success in the magnetron sputtering of magnetic nickel has been achieved by using specially fabricated targets in which a thin layer of the ferromagnetic material plates onto a non-ferromagnetic base material. The layer is thin enough so as not to completely shunt the magnetic field, but again, the target is now very expensive and the source has a severely limited lifetime due to the reduced amount of source material.
The Curie temperature varies over a wide range for various materials. By properly forming an alloy of the ferromagnetic nickel with one or more other elements, the Curie temperature can be reduced from that of pure nickel to a temperature lower than the desired sputtering temperature. For example, Nanis, in U.S. Pat. No. 5,405,646, disclose binary systems of platinum, palladium, molybdenum, vanadium, silicon, titanium, chromium, aluminum, antimony, manganese and zinc. Similarly, Wilson, in U.S. Pat. No. 4,159,909, discloses the use of platinum, copper and tin to render nickel paramagnetic at room temperature. As far as known, targets manufactured from these alloys have not received widespread acceptance in the marketplace.
In addition, adding about 7 weight percent vanadium to nickel lowers the Curie temperature for obtaining paramagnetic properties at room temperature. The Curie temperature is the lowest temperature before spontaneous magnetization occurs. The Curie temperature separates the disordered paramagnetic phase from the ordered ferromagnetic phase. Stated in another way, at temperatures below a material""s Curie temperature, that material is strongly magnetic or ferromagnetic. For temperatures at and above the Curie temperature, the magnetic properties disappear.
With its shift in Curie temperature, Ni-7 V (wt. %) has become the standard composition for use with direct current magnetron sputtering systems to deposit magnetic nickel. Nickel/vanadium (Ni/V) serves as a barrier/adhesion layer for under-bump metals to support flip chips, or C4 (collapsed, controlled, chip connection) assemblies. The flip chips allow high I/O counts, good speed and electrical performance, thermal management, low profile, and the use of standard surface mount and production lines for assembly. Unfortunately, Ni/V target materials are susceptible to high impurity concentrations and to cracking during fabrication of the target blanks. Moreover, Ni/V films can suffer problems during subsequent etching procedures.
Because magnetic nickel is a highly desirable thin film-for many microcircuit and semiconductor device applications, there is a need to develop a method for sputtering high purity magnetic nickel that does not suffer the above disadvantages.
The sputter target deposits nickel from a binary alloy. The binary alloy contains, by weight percent, 9 to 15 titanium and the balance nickel and incidental impurities. The binary alloy has, by weight percent, 35 to 50 TiNi3 needle-like intermetallic phase and balance xcex1-nickel phase. The TiNi3 needle-like intermetallic phase and xcex1-nickel phase are formed from a eutectic decomposition. The xcex1-nickel phase has a grain size between 50 and 180 xcexcm. The binary alloy has a Curie temperature of less than or equal to a temperature of 25xc2x0 C. and exhibits paramagnetic properties at temperatures of 25xc2x0 C. or lower.
The method forms a binary nickel-titanium sputter target blank by first casting a binary alloy of the above composition into an ingot. The binary alloy has, by weight percent, 35 to 50 TiNi3 intermetallic phase and balance xcex1-nickel phase. Then, dissolving the TiNi3 intermetallic phase into a single xcex1-nickel phase at a temperature of at least 1000xc2x0 C. prepares the alloy for hot working. Hot working the ingot at a temperature between 1000xc2x0 C. and the ingot melting temperature forms the target blank, reduces the thickness by at least fifty percent and reduces the xcex1-nickel phase grain size to between 50 and 180 xcexcm. Finally, cooling the target blank precipitates a needle-like TiNi3intermetallic phase in a xcex1-nickel phase matrix to form the final microstructure.
The present invention provides the specific alloying concentration for this binary alloy with a method of preparing targets that facilitates magnetron sputtering of nickel. Magnetron sputtering of nickel can be accomplished by using a binary nickel alloy target material having a properly selected titanium alloying concentration in order that the alloy has a Curie temperature at or below room temperature (25xc2x0 C.), thereby making the material paramagnetic at room temperature.
A series of incremental tests determined that about 9 weight percent was the minimum amount of titanium necessary to make the alloy paramagnetic at room temperature. For purposes of this specification, all composition""s units are expressed in weight percent, unless specifically noted otherwise. This alloy allows the thickness of the target to be increased significantly as compared to a pure nickel target, thereby decreasing the sputtering cost per wafer. With the lower Curie temperature, the alloy is non-ferromagnetic at the sputtering temperature, and is therefore amenable to magnetron sputtering.
Alloying about 9 to 15 weight percent titanium with the balance nickel and incidental impurities produces a sputtering target with paramagnetic properties at room temperature. Advantageously, the alloy contains about 9.5 to 12 weight percent titanium. Most advantageously, the alloy has a nominal composition of about ten weight percent. In addition, advantageously, limiting impurities to less than 0.1 percent provides commercially pure properties. Most advantageously, the target contains less than 0.01 percent impurities.
The melting of the nickel and titanium source material advantageously occurs under a vacuum or protective atmosphere. Most advantageously, a vacuum furnace, such as a semi-continuous vacuum melter (SCVM) can melt the source material in a steel, graphite or ceramic mold. Advantageously, the vacuum is about 1.0xc3x9710xe2x88x924 mTorr to about 10.0 mTorr.
Advantageously, the binary alloy casting occurs under an atmosphere pressure of less than about 5 mTorr. For example, vacuum atmospheres having a pressure of about 1 mTorr to about 5 mTorr are effective for limiting uncontrolled oxidation of the melt. In addition, pouring into molds having a low pressure protective atmosphere is another procedure for limiting oxidation. To maintain low impurities, it is important to cast the alloy under a controlled atmosphere such as under a protective argon, helium or other Group VIII gas or combination of gases. For example, a low pressure argon atmosphere of about 0.1 to about 0.7 atm, such as about 0.3 atm has been found to provide adequate protection to the melt and the ingot upon pouring.
After the molten alloy is cast into a mold, the alloy cools and solidifies into an xe2x80x9cas castxe2x80x9d structure of TiNi3 intermetallic phase and balance xcex1-nickel phase. This as-cast structure is unacceptable for sputtering targets.
To process the alloy, the alloy is first heated for a sufficient period of time and temperature to dissolve the TiNi3 intermetallic phase into an xcex1-nickel phase. If the TiNi3 intermetallic phase remains during deformation, the ingot cracks. Temperatures between about 1000xc2x0 C. and melting are sufficient to dissolve the intermetallic. Most advantageously, a furnace heats the alloy to a temperature between about 1050 and 1150xc2x0 C. In addition, the process advantageously heats the ingot for at least one hour and most advantageously at least two hours, to ensure the dissolution of the intermetallic phase.
After dissolving the intermetallic phase, hot working the ingot with at least a fifty percent reduction in thickness breaks the xcex1-nickel grains into a suitable size. Advantageously, hot working occurs at temperatures between about 1000xc2x0 C. and melting to prevent cracking. Most advantageously, the process hot works the ingot at a temperature between about 1050 and 1150xc2x0 C.
Advantageously, the hot working consists of hot rolling the ingot into a target blank. Most advantageously, the hot rolling is in a single direction to lower the likelihood of cracking. In addition, maintaining the ingot at a temperature between about 1000xc2x0 C. and melting during rolling also serves to hold the intermetallic phase in solution and reduce the likelihood of cracking during rolling. Most advantageously, the hot rolling includes reheating between each rolling pass to maintain temperature-during experimentation, reheating between only every other pass resulted in cracking.
Advantageously, the process relies upon multiple passes with each hot rolling pass being less than about 0.05 inch (1.3 mm). For example, multiple passes of about 0.02-0.05 inch (0.5 to 1.3 mm) are effective. Most advantageously, the reduction per pass is between about 0.5 and 1 mm. In addition, having at least ten reduction passes within this range ensures the production of uniform xcex1-nickel grains.
After hot working, cooling the target blank precipitates about 35 to 50 weight percent needle-like TiNi3 intermetallic phase in an xcex1-nickel phase matrix. Advantageously, the alloy contains about 35 to 45 weight percent needle-like TiNi3 intermetallic phase. Most advantageously, the alloy contains about 38 to 42 weight percent needle-like TiNi3 intermetallic phase. The xcex1-nickel phase grain size is between about 50 and 180 xcexcm. Most advantageously, the grain size is between about 70 and 100 xcexcm. In addition to this relatively small grain size, the alloy advantageously contains relatively equiaxed grains of xcex1-nickel phase. Most advantageously, the xcex1-nickel phase has between 10 and 40 percent of each of the following four crystallographic orientations: (111), (200), (220), and (311). After cooling, machining the target blank produces a sputter target having excellent sputtering characteristics.